Secondary electron emission and charging mechanisms in Auger Electron Spectroscopy and related e-beam techniques

Mechanisms of charging in AES are reconsidered to the light of recent developments concerning the leading role of secondary electron emission (SEE) in the self-regulation processes taking place in insulating materials irradiated with keV-electrons. A specific attention is paid to SE angular distribution and to distortion of SE trajectories by the electric field build-up into the vacuum when the surface potential is negative. These external mechanisms are associated to internal mechanisms resulting from the effect of low potential barrier or of low hollow in the SE generation region. From these investigations the main parameters governing the time dependence of charging have been identified and the fact that the critical energy (between positive and negative charging region at steady state), E2C, is less than the convention critical energy Eo2 (where the total SEE yield is unity) has been re-confirmed for AES. Common points and differences between AES and other electron beam techniques (such as those based on external collector positively biased in SEE yield measurements) are also discussed in detail and possible experimental artefacts are also pointed out (such as those resulting from the incorrect use of the shift of Duane-Hunt limit with a X-ray detector in a SEM). Some practical consequences to minimize charging effects (specimen preparation, operating conditions and use of additional irradiations) have been deduced.